Salinisation is a natural process that results from:
In some areas (for example in Australia), salinity is an inherent situation (enormous amounts of salts are stored in the soils).
However, human practices have increased soil salinity by changing the natural balance of the water cycle, by allowing excess recharging of groundwater and accumulation through concentration.
One of the best examples for excess salination has been observed in Egypt in 1970 when the Assuan dam was built. The change in the level of ground water before the construction had enabled soil erosion, which lead to high concentration of salts in the water table. After the construction, the continuous high level of the water table lead to the salination of the arable land.
Salinity from drylands can occur when table water is between two to three metres of the surface of the soil. The salts from the ground water are raised by capillarity to the surface of the soil. This occurs when groundwater is saline (which is true in many areas), and is favored by land use practices allowing more rainwater to enter the aquifer than this one could accommodate (for example, the clearing of trees for agriculture is a major reason for drylands in some areas, since deep rooting of trees has been replaced by shallow rooting of annual crops).
Salinity from irrigation occurs when water is added and is not used by crops - it is essentially due to over-irrigation, inefficient water use (such as bad estimates of evapotranspiration) and poor drainage, and is also highly favored by use of saline water for irrigating agricultural crops. These practices result in over-concentration of salts.
Salinity in urban areas is often both the combination of over-irrigation and drylands. Cities are often located on drylands, leaving the rich soils for agriculture. Irrigation is also now common management in cities (gardens and recreation areas).
The consequences of salinity are